The present invention relates to alumino earth-alkali silicate glasses for molybdenum-glass fusions in the form of light bulbs as the outer casing for lamps, in particular, for lamps with regenerative halogen cycle and bulb temperatures of from above 550° C. up to 700° C.
It is known that the stability of the regenerative halogen cycle in halogen lamps is the prerequisite for reaching the target lamp life of a halogen lamp. Decisive for this is that the equilibrium between formation and decomposition of tungsten halides is maintained. Disruptions of the halogen cycle, inter alia, can be caused by smallest amounts of contaminations in the glass as well as in the filament material or the feed-through sleeve material. These contaminations, inter alia, can weaken the halogen cycle as a result of the high lamp temperatures as well as the energy-rich radiation of the tungsten filament so that metallic tungsten will form a black precipitate on the inner side of the bulb. This causes a weakening of the lamp efficiency and light translucence. It is a well-known fact that particularly alkali ions have such a disruptive effect on the halogen cycle. For this reason, industrial scale halogen lamp glasses are practically free of alkali, which recently has resulted in alkali oxide contents (R2O) of <0.03% by weight, inasmuch as no stabilizing components partially compensate their effect. In addition to the negative effect of the alkali ions, other components such as H2, OH−, CO and CO2 are said to have an aggressive effect and to cause disruptions of the cycling process.
EP 0 913 366, DE 197 47 354, and U.S. Pat. No. 6,074,969 (same patent family) therefore limit the water contents for aluminoborosilicate glass containing alkaline earth metals employed in lamp bulbs to be operated at temperatures below 650 degrees C. to <0.02% by weight in order to prevent blackening of the lamp.
EP 0 913 365, DE 197 58 481, and U.S. Pat. No. 6,069,100 (same patent family, based on WO 99/14794) discloses aluminoborosilicate glass containing alkaline earth metals for lamp bulbs to be operated at temperatures above 650 degrees C. The references teach that the water contents must be less than 0.02% by weight because the water or the hydrogen ions are said to cause a disruptive effect on the halogen cycling process; discoloration of the glass at the operating temperatures of the lamp is to be prevented in this way.
U.S. Pat. No. 4,163,171 discloses a glass composition which is atypical for halogen lamp glasses (SiO2 50%, P2O5 4.8%, and Al2O3 19.2%) wherein the CO and alkali contents are practically zero and the water contents is limited to less than 0.03% by weight. Glasses of this type of composition however have practically not been used as halogen lamp glasses.
Numerous hart glasses which have been, and are being used, for halogen lamps, for example, glasses 180 made by General Electric; 1720, 1724, and 1725 made by Corning; as well as 8252 and 8253 made by a Schott, have water contents under 0.025% by weight, partially under 0.02% by weight. These glasses are within the composition range according to Table 1.
TABLE 1Oxides% by weightSiO256.4–63.4Al2O314.6–16.7B2O3  0–5.0BaO 7.5–17.0CaO 6.7–12.7MgO  0–8.2SrO  0–0.3ZrO2  0–1.1TiO2  0–0.2Na2O0.02–0.05K2O0.01–0.02Fe2O30.03–0.05
The typical compositions of halogen lamp glasses in patents are within the range of Table 2.
TABLE 2Oxides% by weightSiO252–71 Al2O313–25 B2O3 0–6.5BaO0–17CaO3.5–21  MgO 0–8.3SrO0–10ZrO2 0–5.5R2O  0–0.08(1.2)TiO20–1 Water<0.025
Observing these limits, in particular of the low water contents, poses significant requirements with regard to the employed raw materials as well as the glass melting process, such as, for example:                use of dried raw materials and refuse glass;        water-free raw materials;        increased technical and thus financial expenditure for the apparatus technology and operation of the glass melting apparatus for obtaining melting temperatures above 1,600° C. with a low partial water vapor pressure above the molten glass.        
There presently exists, and there will exist in the future, a significant demand for glasses for halogen lamps.